The present invention relates to a process and apparatus for making high temperature superconducting oxide films, and more particularly to a single-step process for forming the superconducting films without further high temperature annealing treatment. The apparatus is unique in substrate heating, simple in structure and low in cost so as to have valuable applications.
The prior art process for the preparation of superconducting oxide films, such as Y--Ba--Cu--O and Bi--Sr--Ca--Cu--O, comprises growing oxide films on a substrate to obtain non-crystal films by heating at low temperatures or without heating and then annealing the non-crystal films at high temperatures so as to produce the superconducting oxide films. For Y--Ba--Cu--O, the annealing step should be conducted at a temperature of above 850.degree. C. and for Bi--Sr--Ca--Cu--O, it should be at a temperature of above 800.degree. C. Owing to the fact that the annealing step should be operated at a temperature of above 800.degree. C., an additional high temperature furnace is necessary. Since the operating temperature is near the melting point of the materials of the substrate, the surface of the films is rough and not smooth. At the same time, the superconducting film easily reacts with the substrate to destroy the superconductivity of the films. Thus, the selectivity of the materials of the substrate is very limited in view of the consideration of high operating temperature of above 800.degree. C. and of no reaction with superconducting oxide films. Now, useful oxide single crystals include SrTiO.sub.3, YSZ (ZrO.sub.2 added with Y.sub.2 O.sup.3), LaAlO.sub.3, MgO, etc.
In order to overcome these drawbacks, the in-situ preparation of superconducting oxide films has been developed. That is, a substrate is preheated to an adequate temperature so that the atoms of the surface of the substrate have enough energy for diffusion and reaction to form films having good crystals and after cooling, superconducting oxide films can be obtained without any further treatments. Many processes for forming films have been developed to be in-situ processes for making superconducting oxide films, such as sputtering, laser ablation, electron beam deposition, etc. In the preparation of Y--Ba--Cu--O superconducting films, the substrate should be heated to a temperature of about 700 .degree. C. and the oxygen pressure in the deposition chamber should be higher than 0.01 torr. However, problems occur. For instance, the generally used heaters for the substrate such as an infrared ray quartz lamp, wire resistor or metal block resistor are easily damaged to shorten their useful life under an environment of high temperature and high oxygen pressure. Furthermore, it is difficult to exactly measure and control the temperature of the substrate. Particularly, for an infrared ray quartz lamp, the adsorption capability of the substrate material, e.g. single crystal oxide, is low but the capability will greatly increase with the growth of the superconducting oxide film. In other words, the temperature difference of the substrate with or without superconducting film is probably more than 200 .degree. C. under the same power supply. Therefore, the reproducibility of the process for preparation of films is poor. In addition, it is also difficult to obtain an even temperature distribution on large substrates such as a surface having a diameter of 2 inches or more due to high irradiation lose at a high temperature. The manufacture of superconductors having large surface area is therefore influenced. This is a problem for conventional in-situ preparation.
To solve the above problems, the present invention discloses a simple process and apparatus for the preparation of sputtered films. In the apparatus in accordance with the present invention, a sputtering chamber, i.e., a film growing chamber or a deposition chamber is changed to a circular heat-resistant steel tube, quartz tube, or alumina tube, etc., and a sputtering target cathode and a substrate anode are all placed in the deposition chamber. The substrate is heated by a furnace surrounding the outside of the deposition chamber. The superconducting oxide films are grown in-situ by sputtering. Owing to the new apparatus for preparation of superconducting films in accordance with the present invention, the process for preparation of high temperature superconducting oxide films is more convenient than conventional ones. Particularly, in accordance with the invention, superconducting films can be produced in a single-step process.
The characteristics of the present invention also reside in the utilization of a furnace surrounding the outside of the deposition chamber to heat the substrate. The sputtering target is simultaneously heated. Hence, for film growth by a direct current sputtering method, the material of the sputtering target should be stable, specifically not decomposing at temperatures in the range of 700.degree.-900.degree. C., and possess high conductivity. Feenstra et al. have found that YBa.sub.2 Cu.sub.3 O.sub.7-X oxide materials are quite stable and do not decompose under an oxygen partial pressure of 0.1 torr and a temperature of 900.degree. C. Further, from the research of Yamaguchi et al. on the conductivity of YBa.sub.2 Cu.sub.3 O.sub.7-X oxide materials, it is also found that their resistance coefficient is 0.1 .OMEGA.-cm and 1 .OMEGA.-cm at temperatures of 700.degree. C. and 900.degree. C., respectively. The above research shows that the higher the oxygen partial pressure, the more stable the target material YBa.sub.2 Cu.sub.3 O.sub.7-X and the better the conductivity.
Therefore, the process and apparatus in accordance with the present invention do not have the conventional problems, for instance, that the heater is easily damaged and the temperature of substrate is difficult to be determined and controlled, and the structure of the apparatus is simple so as to lower the cost of a deposition system. The present invention can be widely applied to the preparation of the superconducting films for electric devices or instruments and thus the present invention is valuable in industrial production.